1. Technical Field
The present invention relates generally to a gaseous polarization process, and in particular to a method and system for delivering a polarized noble gas to a subject or sample in a manner that minimizes depolarization of the gas during its delivery.
2. Discussion
Polarized noble gases, such as .sup.129 Xe, .sup.3 He, .sup.21 Ne, .sup.83 Kr, and .sup.131 Xe, have a wide variety of present and potential medical and therapeutic applications. For example, when .sup.129 Xe gas is inhaled by a subject, it is transported from the lungs of the subject to the blood and then throughout the body before becoming concentrated in lipids and proteins of tissues such as those forming the lungs and the brain. Studies of surfaces and bulk structures of materials with polarized .sup.129 Xe or other noble gases have also been suggested. Specifically, polarized noble gases provide useful information when used in magnetic resonance imaging (MRI) applications and nuclear magnetic resonance (NMR) studies. Examples of such an application and the results thereof are reported in an article by M. S. Albert, G. D. Cates, D. Driehuys, W. Happer, B. Saam, C. S. Springer, Jr., and A. Wishnia entitled "Biological Magnetic Resonance Imaging Using Laser Polarized .sup.129 Xe", 370 Nature 199 (Jul. 21, 1994).
Known methods of polarizing noble gases incorporate a resonant light source to optically pump an alkali metal vapor. The angular momentum of photons is transferred from the light source to the alkali metal vapor atoms via cyclical resonant absorption or scattering. As alkali metal vapor atoms absorb this angular momentum, the non-polarized noble gas is introduced into the same environment. The optically pumped alkali metal vapor atoms then collide with the non-polarized noble gas atoms and transfer polarization from the alkali metal vapor atoms to the noble gas atoms. These collisions polarize certain noble gas isotopes such as those mentioned above. The alkali metal vapor atoms are typically pumped with one of any number of certain light sources, such as alkali lamps, dye lasers, Ti-sapphire lasers, argon ion lasers, and diode lasers. In addition, the noble gases may be optically pumped by a diode laser array. One particular application of the diode laser array is disclosed in issued U.S. Pat. No. 5,617,860 entitled METHOD AND SYSTEM FOR PRODUCING POLARIZED .sup.129 Xe GAS, which is hereby incorporated by reference. Diode laser arrays have also been used to polarize .sup.3 He as described by M. E. Wagshul and T. E. Chupp in an article entitled "Optical Pumping of High Density Rb with a Broad Band Die Laser and Ga:Al:As Diode Laser Arrays: Application to .sup.3 He Polarization", 40 Physical Review 4447 (1989). Another reference dealing generally with polarization of noble gases includes an article by G. D. Cates, R. J. Fitzgerald, A. S. Barton, P. Bogorad, M. Gatzke, N. R. Newbury and B. Saam entitled "Rb-.sup.129 Xe Spin Exchange Rates Due to Binary and Three-Body Collisions at High Xe Pressures," Physical Review A, Volume 40, Number 8, Oct. 15, 1989, pgs. 4447-4454.
While these references deal generally with the actual polarization of noble gases, the references do not specifically deal with how the gas, once polarized, is stored and delivered to a subject or sample. Polarization lifetimes vary depending on the species and isotope as well as the environmental container. For .sup.129 Xe, polarization lifetimes range from approximately ten minutes in room-temperature glass containers to several hours frozen at liquid nitrogen or lower temperatures. In biological environments, .sup.129 Xe lifetimes are a few to tens of seconds. In addition components such as lasers and specially designed polarization chambers are required to realize the polarization process. As a result, the expense associated with polarizing, storing and delivering noble gases is typically quite high.